This invention relates to apparatus for detecting the direction of a magnetic field and, more particularly, to a magnetoresistive device suitable for detecting angular positions of a rotary element and having ready application in detecting the position of a rotor.
It is a common practice to detect angular positions of a rotor of an electric motor, such as a brushless electric motor, to thereby effect speed and position control. As typical examples, position detectors of this type are used in turntable devices, magnetic recording devices VTR apparatus and the like to attain a high degree of speed and position control of the rotating elements. Other uses are found as, for example, a brushless motor tachometer.
In brushless motor applications, the rotor position can be sensed by a magnetoelectric transducer in order to control the currents flowing into the stator coil of the motor. The magnetoelectric transducer may, for example, be constituted by a semiconductor transducer, such as a semiconductor Hall device, a semiconductor magnetoresistive element or a planar Hall element. Alternatively, the magnetoelectric transducer may be constituted by a ferromagnetic transducer, such as a ferromagnetic magnetoresistive element.
The temperature characteristics of a semiconductor transducer have undesirable effects, since the number and the mobility of the charge carriers vary widely with temperature. Accordingly, when such semiconductor transducers are used, a temperature compensating device is generally required. Moreover, the output signal produced by the semiconductor transducer is dependent upon and thus varies with the intensity of the magnetic field which is sensed. Consequently, if the semiconductor transducer is used as a switching element for detecting the direction of the magnetic field, for example, in brushless motor, additional circuitry must be employed to improve the accuracy of the transducer output and to effect a limiting operation so as to minimize the effects due to field intensity variation. These circuits which must be used to correct and compensate the semiconductor transducers are expensive and complex.
The ferromagnetic transducer, on the other hand, has a temperature characteristic which does not result in the undesirable effects noted above, because the resistivity of the ferromagnetic transducer varies only slightly with temperature. Moreover, when the ferromagnetic transducer is saturated with a magnetic field, the output is relatively insensitive to variations in the intensity of the magnetic field, and thus a self-limiting operation is effected. Consequently, the ferromagnetic transducer is more advantageous for use as a switching element than the semiconductor transducer when the direction of the magnetic field is to be detected. The planar Hall element suffers from the disadvantage that its output voltage is of very low amplitude and thus requires the use of a high gain amplifier to provide necessary voltage amplification. A conventional magnetoresistive element having two terminals has the disadvantage that the unbalanced voltage which is produced in the absence of an applied magnetic field is several orders of magnitude as high as the output voltage which is produced when a magnetic field is detected, although the output voltage is considerably high. Also, drift due to variations in temperature must be compensated.
Some of the aforenoted problems attending the measurement and detection of magnetic fields have been avoided by recently proposed devices. One such proposal is described in U.S. Pat. No. 3,405,355 to Hebbert, which discloses a magnetometer employing thin film magnetic films having magnetoresistive properties. The relationship between the resistivity of the thin film material and the angle of rotation of the magnetization in the film is used to measure external magnetic fields. When a biasing field is applied to the magnetoresistive films, fields of high intensity can be measured.
Another proposal is described in copending Application Ser. No. 487,282, filed July 10, 1974, now U.S. Pat. No. 3,928,836 issued Dec. 23, 1975 and assigned to the same assignee of the present invention. In the copending application a magnetoresistive element is provided wherein ferromagnetic metal film strips are disposed in mutually perpendicular configuration on an insulating substrate so that current flows through these series-connected strips in directions which are predominantly perpendicular to each other. These perpendicular strips may be formed on the same side or on opposite sides of the substrate.
The present invention utilizes the basic teachings of the aforedescribed copending application to attain improved results not heretofore achieved by any of the prior art proposals.